The 4 Ds principle


Classic weathertightness design


Opinion piece: By Flashman Flashings Ltd marketing manager Mike Anticich


In 2004 I was privileged to be shown around several multi-storey leaky apartment blocks in Vancouver, Canada, which were being remediated.


The purpose of the visit was to see first hand how the Canadians were handling the leaky building syndrome since they had already amassed 10 years’ experience in repairing leaky buildings.


A highlight of the trip was meeting with the man who coined the phrase “the 4 Ds” — Vancouver architect Don Hazeldean.


Personally, I find it very helpful to package a concept in a single word or a mnemonic such as the 4 Ds because I find it significantly aids memory and prompts a quick recall of important principles via a bite sized piece of information.


The 4 Ds stand for Deflection, Drainage, Drying and Durability. However, there are still daily breaches of these basic flashing rules that lead to massive costs and huge liability for builders.


I am not at all convinced that builders are aware of their liability when flashings are improperly installed or not installed at all, and which cause a building to leak.


To me the first item Deflection is the most important. If the water is largely deflected then the Drainage and Drying aspects are pretty much taken care of. If the water can be kept out in the first instance then there is nothing to drain or to dry. Basic!


If this is the case, then it is paramount that proper deflective flashings are fitted in the first place to minimise the water that can get in and allow easy paths for any water to get out.


The 4 Ds principle is not something recently discovered. If you look at any old masonry or even large good quality public timber buildings you will observe how the windows and doors are set well back from the exterior wall, and there is at least one, but often several, drip edges that slow the rate of water being blown back to the window or door opening.


In the past I considered this carved stonework or detailed brickwork above windows and doors as purely decorative. That is not its primary purpose!


The decorative detail serves a very important functional purpose. These windows don’t leak. They do this without air seals, flashing tapes, cavities or BRANZ appraisals.


How come? The builders and designers understood basic flashing design. So should we.


The 4Ds principle:


Determine from the outset that any water cannot enter behind or under the flashing

Be aware of severe conditions and high winds or exposure

Can any further protection be added?

Don’t ever rely on sealant alone — who will maintain and replace the sealant?

Don’t ever rely on paint alone — what happens if the paint fails or wears? Is their a back-up?

Is it possible to re-direct the water away from vulnerable points? — eg, an upper roof downpipe dispersing water next to a valley or apron flashing junction could be extended and fitted with a downpipe spreader.

Always use a cavity! I cannot understand why anyone would not use a cavity. The cost to fit battens is only about $1000 for a standard new house which buys 50 years of the cheapest insurance.



Cavity construction alone is not enough to ensure against leaks. Correct and appropriate flashings are still required to prevent leaks with cavity construction. The first batch of leaky buildings with cavity construction has arrived.

A cavity is not to be used as an internal downpipe!

A flashing which is designed to let water in, such as a valley flashing, must also be designed to let water out. This is basic but last week I spoke to a man who had a severe leak in one of his bedrooms simply because the bottom of the valley was finished approximately 100mm short of the spouting.

The house is about four years old so has been leaking since construction. Only the continuous rain of the previous week highlighted the exact location. Do you think there might be some damage? I know this sounds basic but it almost always is!


You cannot expect sealant to do the job of a metal flashing. If water does get in around the window or door it must get out. Do not rely on a cavity which was only ever meant to be a back-up in extreme conditions.



Drying is achieved by four mechanisms as listed below with their approximate drying times.

Drainage: Hours

Ventilation: Days to weeks

Capillary transfer, ie, wicking: Weeks

Diffusion, ie, vapour transfer: Months

It can be clearly seen therefore that drainage is the best option. Check that you have good drainage in flashings that are designed to let water in and then make certain that water is directed out again. This is vital for valleys and at the end of apron flashings where they enter the spouting.


The lack of apron diverters is the cause of huge damage in buildings, and can be so easily avoided.



The NZ Building Code only requires 15-year durability for the external envelope — ie, roof, walls, windows, doors and cladding. Flashings at all these elements also require a 15-year life.

Make sure then that the flashings you are using will meet the 15-year minimum durability requirement.

Recently I was asked to look at a roof that had been fitted with a flexible flashing tape that was clearly only suitable for interior or fully protected use. It had failed in days but was applied by contractors who insisted it was ideal for the purpose. The judge, I believe, will see it very differently.



I consider that it is impossible to achieve, in practice, many of the drawings that are proposed as answers to common flashing problems.


This is why I believe there must be a fifth D added to the existing 4 Ds — Do-Ability.


Unless the flashing solution is simple, quick and easy to install on site, cost effective and able to be fitted successfully in all manner of situations, it is a waste of time drawing it.

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